Microporous membranes are well known to those skilled in the art. These membranes are fabricated from organic polymers as thin films or hollow fibers with continuous networks of interconnected pores leading from one surface to the other. The rate at which solvent, ions, monomer and polymer molecules, and other small particles pass through the pores depends not only on pore size but also on mutual attractions and repulsions between the membrane material and the materials in the pores. These membranes can be used for the separation of very small particles, such as colloids and polymers, from each other or from the liquid in which they are suspended. They are also useful as separators in rechargable batteries, wherein two electrodes must be physically separated from each other in such a way that there is a continuous pathway for exchange of small ions without the mixing of reactants and products of the two half-cell reactions. These types of membranes can also be used in applications in which gas diffusion is desired, as in blood oxygenators, wherein the membrane has a liquid in contact with one surface and a gas in contact with the other surface. Other biological and medical applications of microporous membranes include bioreactors, in which the membrane provides a medium in which a biologically active substance can act on a solution, converting reactants to products without the biologically active substance becoming dispersed in the product solution; and blood dialysis, where waste products are removed from blood. Finally, microporous membranes have been used as supports for liquid membranes, wherein a liquid which is imbibed in the pores of the microporous membrane is the medium through which transport takes place. Stable liquid membranes are inherently difficult to make.
Microporous membranes are made from organic polymers by a variety of different methods. Organic polymers that are currently used to make microporous membranes include cellulose esters, as for example cellulose acetate; polyvinyl chloride; polysulfones and other high temperature aromatic polymers; polytetrafluoroethylene; polyolefins, including polypropylene and polyethylene; polycarbonates; polystyrene; and nylons. Because of the complexity of fabricating polymers into microporous membranes, it is often difficult to select a polymer for a specific application based on the physical and surface properties of the polymer and then fabricate that polymer into a membrane. This most often is the case when water wettability is desired, as is usually true for biological applications. Most organic polymers are not wetted by water. Many polymers that are wetted by water, as for example, polyacrylic acid, have physical properties that make fabrication of microporous membranes impractical. Therefore, practitioners in the art have strived to develop methods to modify the surface properties and especially water wettability of the readily available microporous membranes.
One method of modification of the surface behavior of a microporous membrane is treatment with a surfactant. As an example, Hoechst Celanese Corporation, the assignee of the present invention, manufactures and sells polypropylene microporous membranes under the name Celgard.RTM., a registered trademark of Hoechst Celanese Corporation. While polypropylene is not wetted by water, some of the commercial Celgard.RTM. products are wetted by water because they have been treated with surfactants. A second approach that has been described in the patent literature is the application of thin films of polymers to microporous membranes. This is most simply done by treatment of a microporous membrane with a polymer solution, followed by removal of the solvent. In both of the approaches described above, loss of wettability can occur due to dissolution or leaching of surfactant or polymer.
A third general method that is described in the literature involves the covalent grafting of monomeric or polymeric materials onto the surface of the membrane by chemical treatment. For example, European Patent Application 302,650 describes a method for converting a hydrophobic polyolefin hollow fiber microporous membrane to a hydrophilic membrane by the grafting of polyvinyl alcohol onto the inner and outer surfaces. The method consists of irradiation of the hollow fiber with ionizing radiation, followed by reaction with vinyl acetate and then hydrolysis. U.S. Pat. No. 4,340,482 teaches a method for converting a polyvinylidene fluoride microporous membrane to a hydrophilic membrane by treatment with a highly basic solution of glycine, whereby the glycine is grafted onto the surface of the membrane.
A fourth method of modifying membranes involves reactions of monomers or oligomers with other monomers that have highly reactive functional groups. This leads to polymerization or crosslinking. For example, U.S. Pat. No. 3,744,642 describes a reverse osmosis membrane that is made by the interfacial condensation of a diamine and a diacid chloride within a porous substrate made of paper, glass fibers, or polymeric fibers, yielding a composite polyamide membrane. U.S. Pat. Nos. 3,951,815, 4,039,440, and 4,337,154 all are directed to the synthesis of composite reverse osmosis membranes by the crosslinking of amine containing polymers within a porous substrate. Although the polymerizations are carried out in porous substrates, the resulting membranes are not in general microporous.
The above-described approach has been further extended to include polymerizations and crosslinking reactions in the pores of microporous membranes in order to entrap the water soluble polymers within the pore networks, thus rendering the hydrophobic membranes hydrophilic. U.S. Pat. No. 4,113,912 teaches that a fluorocarbon microporous membrane, such as polyvinylidene fluoride, can be made hydrophilic by filling the pores with an aqueous solution of a water-soluble polymer, as for example polyacrylic acid, polyacrylamide, or polyvinyl alcohol, and then subjecting the polymer-treated membrane to reagents and conditions that lead to insolubilization of the polymer, generally by crosslinking. European Patent Application 257,635 teaches that hydrophobic membranes, with fluorocarbon membranes used as examples, can be rendered hydrophilic by filling the pores with an aqueous solution containing one or more hydrophilic polyfunctional amine- or hydroxy-containing monomers or polymers, such as water-soluble cellulose derivatives or polyvinyl alcohol, along with crosslinking agents and optional catalysts, surfactants and initiators. The solutions described above are formulated with the goals of improving penetration of the pores and also of inducing crosslinking to take place or causing the hydrophilic compound to chemically bind to the fluorocarbon substrate.
It is an object of the present invention to provide a broadly applicable method for modifying microporous membranes by the polymerization of vinyl monomers that are incorporated in the porous network of the membrane in organic or aqueous solvents to a high enough molecular weight that they can't be removed from the pores, thereby producing stable pore-modified membranes.
It is a further object of the present invention to provide liquid membranes that are stable under conditions of use. Still further objects of this invention are to provide stable battery separators and stable hydrophilic bioreactors that use the pore-modified structure of the current invention.
These and other objects as well as the scope, nature and utility of the invention will be apparent to those skilled in the art from the following Detailed Description and Appended claims.